This also means that rb-pinning-service-api is no longer required for
running remote pinning tests. This alone saves at least 3 minutes in
test runtime in CI because we don't need to checkout the repo, build
the Docker image, run it, etc.
Instead this implements a simple pinning service in Go that the test
runs in-process, with a callback that can be used to control the async
behavior of the pinning service (e.g. simulate work happening
asynchronously like transitioning from "queued" -> "pinning" ->
"pinned").
This also adds an environment variable to Kubo to control the MFS
remote pin polling interval, so that we don't have to wait 30 seconds
in the test for MFS changes to be repinned. This is purely for tests
so I don't think we should document this.
This entire test suite runs in around 2.5 sec on my laptop, compared to
the existing 3+ minutes in CI.
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